In recent years, the rapid expansion of the world's population coupled with the accelerated technological development of large sectors of the world has produced a dramatic increase in the demand for energy in all forms including fuels and electricity for heating, lighting, transportation and manufacturing processes. The construction of hydroelectric facilities and the development of fossil fuel resources has continued at a rapid rate, but it becomes increasingly evident for a number of reasons that these efforts are inadequate to keep pace with the demands of the growing population.
One of the more challenging problems to be confronted in the harnessing of a solar energy source is the development of a suitable means for converting thermal energy to mechanical or electrical energy.
The steam turbine is frequently proposed for this purpose, but is not ideally suited for a number of reasons. The first is that it requires high operating temperatures and a high temperature differential from input to exhaust. This imposes difficulties in construction and the high temperatures are difficult to accomplish in a relatively small solar system without sustaining excessive thermal energy losses. The small steam turbine is also inherently inefficient, particularly if it is not operated consistently at optimum conditions of temperature, velocity and load. Furthermore, the construction of a practical steam turbine is too complex and too expensive for all but very high power ratings. Cooling water needs are untenable.
What is needed is a conversion means which is operable at relatively low temperatures and which is efficient over a wide range of operating power levels. It should be simple in construction to permit economy at low power levels and it should offer high reliability and low maintenance.
The typical turbine allows the gas to expand as it ricochets between fixed and revolving sets of blades. The change of directions at each blade causes the kinetic energy of the gas velocity to impart moment to the revolving blades thus creating shaft energy. High vapor velocity and high peripheral blade speeds are required for maximum efficiency. Maximum torque is developed near operating R.P.M. As there is no positive displacement effect, non-productive flow (slip) is approximately inversely proportional to the R.P.M. At stalling speed down to "locked rotor" the "slip" becomes 100%. At stall, a small torque is apparent but no useful work is done, even at full flow.
Multistage turbines having many ranks of fixed and moving blades have a temperature gradient spread over the total path of the vapor. From the superheated inlet to the condenser tubes the greater the .DELTA..sup.t temperature (difference) the higher the efficiency. Maximum vapor volume and velocity is created by the vacuum of condensation. To take advantage of this increase, the blades of each row are longer and larger in diameter than the preceding row. For maximum economy, the exit vapor from the first stage is often returned to the boiler for re-superheating (to add additional energy for greater velocity and expansion).
Due to the enormous quantity of heat absorbed by the cooling water of the condensers (usually greater than 1,000 BTU per lb. of steam) high efficiency can only be obtained with very high temperatures (1000.degree. F.) and pressures (2500-3000 PSI). These are only practicable in super powered plants (larger than 50,000 K.W.). Usually the overall thermal efficiency of these large installations seldom exceed 42% of the total fuel energy converted to electrical power.